BMP file format
The BMP file format, also known as bitmap image file, device independent bitmap file format and bitmap, is a raster graphics image file format used to store bitmap digital images, independently of the display device, especially on Microsoft Windows and OS/2 operating systems.
The BMP file format is capable of storing two-dimensional digital images both monochrome and color, in various color depths, and optionally with data compression, alpha channels, and color profiles. The Windows Metafile specification covers the BMP file format.
Device-independent bitmaps and the BMP file format
Microsoft has defined a particular representation of color bitmaps of different color depths, as an aid to exchanging bitmaps between devices and applications with a variety of internal representations. They called these device-independent bitmaps or DIBs, and the file format for them is called DIB file format or BMP image file format.According to Microsoft support:
A device-independent bitmap is a format used to define device-independent bitmaps in various color resolutions. The main purpose of DIBs is to allow bitmaps to be moved from one device to another. A DIB is an external format, in contrast to a device-dependent bitmap, which appears in the system as a bitmap object. A DIB is normally transported in metafiles, BMP files, and the Clipboard.
The following sections discuss the data stored in the BMP file or DIB in detail. This is the standard BMP file format. Some applications create bitmap image files which are not compliant with the Microsoft documentation. Also, not all fields are used; a value of 0 will be found in these unused fields.
File structure
The bitmap image file consists of fixed-size structures as well as variable-sized structures appearing in a predetermined sequence. Many different versions of some of these structures can appear in the file, due to the long evolution of this file format.Referring to the diagram 1, the bitmap file is composed of structures in the following order:
| Structure name | Optional | Size | Purpose | Comments |
| Bitmap file header | 14 bytes | To store general information about the bitmap image file | Not needed after the file is loaded in memory | |
| DIB header | Fixed-size | To store detailed information about the bitmap image and define the pixel format | Immediately follows the Bitmap file header | |
| Extra bit masks | 3 or 4 DWORDs | To define the pixel format | Present only in case the DIB header is the and the Compression Method member is set to either BI_BITFIELDS or BI_ALPHABITFIELDS | |
| Color table | To define colors used by the bitmap image data | Mandatory for color depths ≤ 8 bits | ||
| Gap1 | Structure alignment | An artifact of the File offset to Pixel array in the Bitmap file header | ||
| Pixel array | To define the actual values of the pixels | The pixel format is defined by the DIB header or Extra bit masks. Each row in the Pixel array is padded to a multiple of 4 bytes in size | ||
| Gap2 | Structure alignment | An artifact of the ICC profile data offset field in the DIB header | ||
| ICC color profile | To define the color profile for color management | Can also contain a path to an external file containing the color profile. When loaded in memory as "non-packed DIB", it is located between the color table and Gap1. |
DIBs in memory
A bitmap image file loaded into memory becomes a DIB data structure – an important component of the Windows GDI API. The in-memory DIB data structure is almost the same as the BMP file format, but it does not contain the 14-byte bitmap file header and begins with the DIB header. For DIBs loaded in memory, the color table can also consist of 16-bit entries that constitute indexes to the currently realized palette, instead of explicit RGB color definitions. In all cases, the pixel array must begin at a memory address that is a multiple of 4 bytes. In non-packed DIBs loaded in memory, the optional color profile data should be located immediately after the color table and before the gap1 and pixel array.When the size of gap1 and gap2 is zero, the in-memory DIB data structure is customarily referred to as "packed DIB" and can be referred to by a single pointer pointing to the beginning of the DIB header. In all cases, the pixel array must begin at a memory address that is a multiple of 4 bytes. In some cases it may be necessary to adjust the number of entries in the color table in order to force the memory address of the pixel array to a multiple of 4 bytes. For "packed DIBs" loaded in memory, the optional color profile data should immediately follow the pixel array, as depicted in diag. 1.
"Packed DIBs" are required by Windows clipboard API functions as well as by some Windows patterned brush and resource functions.
Bitmap file header
This block of bytes is at the start of the file and is used to identify the file. A typical application reads this block first to ensure that the file is actually a BMP file and that it is not damaged. The first 2 bytes of the BMP file format are the character "B" then the character "M" in ASCII encoding. All of the integer values are stored in little-endian format.| Offset hex | Offset dec | Size | Purpose |
| 00 | 0 | 2 bytes | The header field used to identify the BMP and DIB file is 0x42 0x4D in hexadecimal, same as BM in ASCII. The following entries are possible:;BM: Windows 3.1x, 95, NT,... etc. ;BA: OS/2 struct bitmap array ;CI: OS/2 struct color icon ;CP: OS/2 const color pointer ;IC: OS/2 struct icon ;PT: OS/2 pointer |
| 02 | 2 | 4 bytes | The size of the BMP file in bytes |
| 06 | 6 | 2 bytes | Reserved; actual value depends on the application that creates the image |
| 08 | 8 | 2 bytes | Reserved; actual value depends on the application that creates the image |
| 0A | 10 | 4 bytes | The offset, i.e. starting address, of the byte where the bitmap image data can be found. |
DIB header (bitmap information header)
This block of bytes tells the application detailed information about the image, which will be used to display the image on the screen. The block also matches the header used internally by Windows and OS/2 and has several different variants. All of them contain a dword field, specifying their size, so that an application can easily determine which header is used in the image. The reason that there are different headers is that Microsoft extended the DIB format several times. The new extended headers can be used with some GDI functions instead of the older ones, providing more functionality. Since the GDI supports a function for loading bitmap files, typical Windows applications use that functionality. One consequence of this is that for such applications, the BMP formats that they support match the formats supported by the Windows version being run. See the table below for more information.| Size | Header name | OS support | Features | Written by |
| 12 | Windows 2.0 or later OS/2 1.x | |||
| 64 | OS/2 2 | Adds halftoning. Adds RLE and Huffman 1D compression. | ||
| 16 | This variant of the previous header contains only the first 16 bytes and the remaining bytes are assumed to be zero values. An example of such a case is the graphic of the BMP Suite. | |||
| 40 | Windows NT, 3.1x or later | Adds 16 bpp and 32 bpp formats. Adds RLE compression. | ||
| 52 | Undocumented | Adds RGB bit masks. | Adobe Photoshop | |
| 56 | Not officially documented, but this documentation was posted on Adobe's forums, by an employee of Adobe with a statement that the standard was at one point in the past included in official MS documentation | Adds alpha channel bit mask. | Adobe Photoshop | |
| 108 | Windows NT 4.0, 95 or later | Adds color space type and gamma correction | ||
| 124 | Windows NT 5.0, 98 or later | Adds ICC color profiles | The GIMP |
The Windows 2.x BITMAPCOREHEADER differs from the OS/2 1.x BITMAPCOREHEADER in the one detail that the image width and height fields are signed integers, not unsigned.
Versions after only add fields to the end of the header of the previous version.
For example: adds fields to, and adds fields to.
An integrated alpha channel has been introduced with the undocumented and with the documented and is used within Windows XP logon and theme system as well as Microsoft Office ; it is supported by some image editing software, such as Adobe Photoshop since version 7 and Adobe Flash since version MX 2004. It is also supported by GIMP, Google Chrome, Microsoft PowerPoint and Microsoft Word.
For compatibility reasons, most applications use the older DIB headers for saving files. With OS/2 no longer supported after Windows 2000, for now the common Windows format is the header. See next table for its description. All values are stored as unsigned integers, unless explicitly noted.
| Offset | Offset | Size | Windows |
| 0E | 14 | 4 | the size of this header, in bytes |
| 12 | 18 | 4 | the bitmap width in pixels |
| 16 | 22 | 4 | the bitmap height in pixels |
| 1A | 26 | 2 | the number of color planes |
| 1C | 28 | 2 | the number of bits per pixel, which is the color depth of the image. Typical values are 1, 4, 8, 16, 24 and 32. |
| 1E | 30 | 4 | the compression method being used. See the next table for a list of possible values |
| 22 | 34 | 4 | the image size. This is the size of the raw bitmap data; a dummy 0 can be given for BI_RGB bitmaps. |
| 26 | 38 | 4 | the horizontal resolution of the image. |
| 2A | 42 | 4 | the vertical resolution of the image. |
| 2E | 46 | 4 | the number of colors in the color palette, or 0 to default to 2n |
| 32 | 50 | 4 | the number of important colors used, or 0 when every color is important; generally ignored |
The compression method can be:
| Value | Identified by | Compression method | Comments |
| 0 | BI_RGB | Most common | |
| 1 | BI_RLE8 | RLE 8-bit/pixel | Can be used only with 8-bit/pixel bitmaps |
| 2 | BI_RLE4 | RLE 4-bit/pixel | Can be used only with 4-bit/pixel bitmaps |
| 3 | BI_BITFIELDS | : Huffman 1D | : RGB bit field masks, +: RGBA |
| 4 | BI_JPEG | : RLE-24 | +: JPEG image for printing |
| 5 | BI_PNG | +: PNG image for printing | |
| 6 | BI_ALPHABITFIELDS | RGBA bit field masks | only Windows CE 5.0 with.NET 4.0 or later |
| 11 | BI_CMYK | only Windows Metafile CMYK | |
| 12 | BI_CMYKRLE8 | RLE-8 | only Windows Metafile CMYK |
| 13 | BI_CMYKRLE4 | RLE-4 | only Windows Metafile CMYK |
An OS/2 2.x contains 24 additional bytes:
| Offset | Offset | Size | OS/2 |
| 36 | 54 | 2 | An enumerated value specifying the units for the horizontal and vertical resolutions. The only defined value is 0, meaning pixels per metre |
| 38 | 56 | 2 | Padding. Ignored and should be zero |
| 3A | 58 | 2 | An enumerated value indicating the direction in which the bits fill the bitmap. The only defined value is 0, meaning the origin is the lower-left corner. Bits fill from left-to-right, then bottom-to-top. Note that Windows bitmaps can also specify an upper-left origin by using a negative value for the image height |
| 3C | 60 | 2 | An enumerated value indicating a halftoning algorithm that should be used when rendering the image. |
| 40 | 64 | 4 | Halftoning parameter 1 |
| 44 | 68 | 4 | Halftoning parameter 2 |
| 48 | 72 | 4 | An enumerated value indicating the color encoding for each entry in the color table. The only defined value is 0, indicating RGB. |
| 4C | 76 | 4 | An application-defined identifier. Not used for image rendering |
The halftoning algorithm can be:
| Value | Halftoning algorithm | Comments |
| 0 | Most common | |
| 1 | Error diffusion | Halftoning parameter 1 is the percentage of error damping. 100 indicates no damping. 0 indicates that errors are not diffused |
| 2 | PANDA: Processing Algorithm for Noncoded Document Acquisition | Halftoning parameters 1 and 2 represent the X and Y dimensions, in pixels, respectively, of the halftoning pattern used |
| 3 | Super-circle | Halftoning parameters 1 and 2 represent the X and Y dimensions, in pixels, respectively, of the halftoning pattern used |
Color table
The color table occurs in the BMP image file directly after the BMP file header, the DIB header. Therefore, its offset is the size of the plus the size of the DIB header.Note: On Windows CE the header can be used with the BI_ALPHABITFIELDS option in the biCompression member.
The number of entries in the palette is either 2n or a smaller number specified in the header. In most cases, each entry in the color table occupies 4 bytes, in the order blue, green, red, 0x00. This is indexed in the under the function biBitCount.
The color table is a block of bytes listing the colors used by the image. Each pixel in an indexed color image is described by a number of bits which is an index of a single color described by this table. The purpose of the color palette in indexed color bitmaps is to inform the application about the actual color that each of these index values corresponds to. The purpose of the color table in non-indexed bitmaps is to list the colors used by the bitmap for the purposes of optimization on devices with limited color display capability and to facilitate future conversion to different pixel formats and paletization.
The colors in the color table are usually specified in the 4-byte per entry RGBA32 format. The color table used with the OS/2 uses the 3-byte per entry RGB24 format. For DIBs loaded in memory, the color table can optionally consist of 2-byte entries – these entries constitute indexes to the currently realized palette instead of explicit RGB color definitions.
Microsoft does not disallow the presence of a valid alpha channel bit mask in and for 1bpp, 4bpp and 8bpp indexed color images, which indicates that the color table entries can also specify an alpha component using the RGBAX|8.8.8.. format via the RGBQUAD.rgbReserved member. However, some versions of Microsoft's documentation disallow this feature by stating that the RGBQUAD.rgbReserved member "must be zero".
As mentioned above, the color table is normally not used when the pixels are in the 16-bit per pixel format ; there are normally no color table entries in those bitmap image files. However, the Microsoft documentation specifies that for 16bpp, the color table can be present to store a list of colors intended for optimization on devices with limited color display capability, while it also specifies, that in such cases, no indexed palette entries are present in this Color Table. This may seem like a contradiction if no distinction is made between the mandatory palette entries and the optional color list.
Pixel storage
The bits representing the bitmap pixels are packed in rows. The size of each row is rounded up to a multiple of 4 bytes by padding.For images with height above 1, multiple padded rows are stored consecutively, forming a Pixel Array.
The total number of bytes necessary to store one row of pixels can be calculated as:
The total number of bytes necessary to store an array of pixels in an n bits per pixel image, with 2n colors, can be calculated by accounting for the effect of rounding up the size of each row to a multiple of 4 bytes, as follows:
Pixel array (bitmap data)
The pixel array is a block of 32-bit DWORDs, that describes the image pixel by pixel. Usually pixels are stored "bottom-up", starting in the lower left corner, going from left to right, and then row by row from the bottom to the top of the image. Unless is used, uncompressed Windows bitmaps also can be stored from the top to bottom, when the Image Height value is negative.In the original OS/2 DIB, the only four legal values of color depth were 1, 4, 8, and 24 bits per pixel.
Contemporary DIB Headers allow pixel formats with 1, 2, 4, 8, 16, 24 and 32 bits per pixel. GDI+ also permits 64 bits per pixel.
Padding bytes must be appended to the end of the rows in order to bring up the length of the rows to a multiple of four bytes. When the pixel array is loaded into memory, each row must begin at a memory address that is a multiple of 4. This address/offset restriction is mandatory only for Pixel Arrays loaded in memory. For file storage purposes, only the size of each row must be a multiple of 4 bytes while the file offset can be arbitrary. A 24-bit bitmap with Width=1, would have 3 bytes of data per row and 1 byte of padding, while Width=2 would have 6 bytes of data and 2 bytes of padding, Width=3 would have 9 bytes of data and 3 bytes of padding, and Width=4 would have 12 bytes of data and no padding.
Compression
- Indexed color images may be compressed with 4-bit or 8-bit RLE or Huffman 1D algorithm.
- OS/2 2 24bpp images may be compressed with the 24-bit RLE algorithm.
- The 16bpp and 32bpp images are always stored uncompressed.
- Note that images in all color depths can be stored without compression if so desired.
Pixel format
- The 1-bit per pixel format supports 2 distinct colors,. The pixel values are stored in each bit, with the first pixel in the most-significant bit of the first byte. Each bit is an index into a table of 2 colors. An unset bit will refer to the first color table entry, and a set bit will refer to the last color table entry.
- The 2-bit per pixel format supports 4 distinct colors and stores 4 pixels per 1 byte, the left-most pixel being in the two most significant bits. Each pixel value is a 2-bit index into a table of up to 4 colors.
- The 4-bit per pixel format supports 16 distinct colors and stores 2 pixels per 1 byte, the left-most pixel being in the more significant nibble. Each pixel value is a 4-bit index into a table of up to 16 colors.
- The 8-bit per pixel format supports 256 distinct colors and stores 1 pixel per 1 byte. Each byte is an index into a table of up to 256 colors.
- The 16-bit per pixel format supports 65536 distinct colors and stores 1 pixel per 2-byte WORD. Each WORD can define the alpha, red, green and blue samples of the pixel.
- The 24-bit pixel format supports 16,777,216 distinct colors and stores 1 pixel value per 3 bytes. Each pixel value defines the red, green and blue samples of the pixel. Specifically, in the order: blue, green and red.
- The 32-bit per pixel format supports 4,294,967,296 distinct colors and stores 1 pixel per 4-byte DWORD. Each DWORD can define the alpha, red, green and blue samples of the pixel.
The sample fields defined by the BITFIELDS bit masks have to be contiguous and non-overlapping, but the order of the sample fields is arbitrary. The most ubiquitous field order is: Alpha, Blue, Green, Red. The red, green and blue bit masks are valid only when the Compression member of the DIB header is set to BI_BITFIELDS. The alpha bit mask is valid whenever it is present in the DIB header or when the Compression member of the DIB header is set to BI_ALPHABITFIELDS.
RGB video subtypes
The BITFIELD mechanism described above allows for the definition of tens of thousands different pixel formats, however only several of them are used in practice, all palettized formats RGB8, RGB4, and RGB1 and:| R.G.B.A.X | RGB subtype | R.G.B.A.X | ARGB subtype |
| 8.8.8.0.8 | RGB32 | 8.8.8.8.0 | ARGB32 |
| 10.10.10.2.0 | A2R10G10B10 | ||
| 8.8.8.0.0 | RGB24 | 10.10.10.2.0 | A2B10G10R10 |
| 5.6.5.0.0 | RGB565 | 4.4.4.4.0 | ARGB4444 |
| 5.5.5.0.1 | RGB555 | 5.5.5.1.0 | ARGB1555 |
In version 2.1.4 FFmpeg supported the BMP pixel formats bgra, bgr24, rgb565le, rgb555le, rgb444le, rgb8, bgr8, rgb4_byte, bgr4_byte, gray, pal8, and monob; i.e., bgra was the only supported pixel format with transparency.
Example 1
Following is an example of a 2×2 pixel, 24-bit bitmap with pixel format RGB24.Example 2
Following is an example of a 4×2 pixel, 32-bit bitmap with opacity values in the alpha channel with pixel format ARGB32.Note that the bitmap data starts with the lower left hand corner of the image.
Usage of BMP format
The simplicity of the BMP file format, and its widespread familiarity in Windows and elsewhere, as well as the fact that this format is relatively well documented and free of patents, makes it a very common format that image processing programs from many operating systems can read and write. ICO and CUR files contain bitmaps starting with a BITMAPINFOHEADER.Many older graphical user interfaces used bitmaps in their built-in graphics subsystems; for example, the Microsoft Windows and OS/2 platforms' GDI subsystem, where the specific format used is the Windows and OS/2 bitmap file format, usually named with the file extension of
.BMP.While most BMP files have a relatively large file size due to lack of any compression, many BMP files can be considerably compressed with lossless data compression algorithms such as ZIP because they contain redundant data. Some formats, such as RAR, even include routines specifically targeted at efficient compression of such data.